Preformed solder parts coated with parylene in a thickness effective to exhibit predetermined interference colors

Abstract

The invention provides a solder preform coated with a predetermined thickness of parylene which physically and chemically protects the preform, as well as a provides a unique optical interference coating which reflects at characteristic frequencies. When predetermined thicknesses of parylene are selected, the parylene coating provides an interference coating which causes the solder preforms to appear green, or gold, or blue in color. The parylene coating, which is otherwise colorless and transparent thus enables solder preforms to be visually distinguishable by alloy type (or customer) by the use of colors, or blends of colors. The invention further provides an effective method of visually verifying the reflow status of a solder preform during manufacturing by examining the color of the solder preform after heating. The interference colors provided by the parylene coating do not persist through the reflow process, and thus, the change in color of the solder preform after reflow can be used as an effective visual check to determine if the preform has been heated sufficiently.

We claim: 
     
       1. A coated solder preform comprising a solder preform coated with a predetermined thickness of a transparent material effective to exhibit an interference color, said transparent material having a melting point lower than the melting point of said solder preform, wherein said transparent material is a labile material of low viscosity at 180° C. or a liquid of low viscosity at 180° C. 
     
     
       2. The coated solder preform of claim 1, wherein said transparent material comprises a poly-p-xylylene material. 
     
     
       3. The coated solder preform of claim 2, wherein said poly-p-xylylene material has a melting point below 180° C. 
     
     
       4. The coated solder preform of claim 3, wherein said poly-p-xylylene material comprises 4-ethyl-(2,2)paracylophane. 
     
     
       5. The coated solder preform of claim 2 wherein said predetermined thickness is in the range of about 50 nm to about 1000 nm. 
     
     
       6. The coated solder preform of claim 5 wherein said predetermined thickness is in the range of about 60 nm to about 700 nm. 
     
     
       7. A method of determining the reflow status of a solder preform during the manufacture of an electrical or electro-mechanical joint comprising the steps of: examining the color of a solder preform coated with a predetermined thickness of a poly-p-xylylene material effective to exhibit an interference color prior to heating the solder preform, wherein said poly-p-xylylene material is a labile material of low viscosity at 180° C. or a liquid of low viscosity at 180° C.; and   examining the color of the reflowed solder material after heating whereby the reflow status of the solder preform is determined.   
     
     
       8. The method of claim 7 wherein said examination is a visual examination of the color of the solder preform. 
     
     
       9. A coated solder preform comprising a solder preform having a surface with a coating of flux on said surface and a predetermined thickness of a transparent material effective to exhibit an interference color coated on said coating of flux, said transparent material having a melting point lower than the melting point of said solder preform, wherein said transparent material is a labile material of low viscosity at 180° C. or a liquid of low viscosity at 180° C. 
     
     
       10. The coated solder preform of claim 9 wherein said predetermined thickness of a transparent material comprises a poly-p-xylylene material. 
     
     
       11. A method of distinguishing a first type of solder preform from a second type of solder preform, comprising the steps of: coating the first type of solder preform with a predetermined thickness of a transparent material effective to exhibit an interference color which produces a first color by destructive interference, said first color distinguishable from the second type of solder preform having a second color, said transparent material having a melting point lower than the melting point of said first type of solder preform, wherein said transparent material is labile or liquid of low viscosity at 180° C.   
     
     
       12. The method of distinguishing types of solder preforms of claim 11, wherein said transparent material comprises a poly-p-xylylene material. 
     
     
       13. The method of distinguishing types of solder preforms of claim 12, wherein said poly-p-xylylene material has a melting point below 180° C. 
     
     
       14. The method of distinguishing types of solder preforms of claim 13, wherein said poly-p-xylylene is 4-ethyl-(2,2) paracylophane. 
     
     
       15. The method of claim 12, wherein said predetermined thickness is in the range of about 50 nm to about 1000 nm. 
     
     
       16. The method of claim 15, wherein said predetermined thickness is in the range of 60 nm to about 700 nm. 
     
     
       17. A method of distinguishing a first type of solder preform from a second type of solder preform, comprising the steps of: applying a coating of flux on the first type of solder preform,   coating onto said coating of flux of said first type of solder preform a predetermined thickness of a transparent material effective to exhibit an interference color which produces a first color by destructive interference, said first color distinguishable from the second type of solder preform having a second color, said transparent material having a melting point lower than the melting point of said first type of solder preform, wherein said transparent material is a labile material of low viscosity at 180° C. or a liquid of low viscosity at 180° C.   
     
     
       18. The method claim 17, wherein said transparent material comprises a poly-p-xylylene. 
     
     
       19. The method of claim 18, wherein said poly-p-xylylene is 4-ethyl-(2,2) paracylophane. 
     
     
       20. A method of coloring a solder preform comprising: using a refractive index of a transparent material to calculate and thereby select a predetermined thickness of said transparent material effective to exhibit an interference color by destructive interference, wherein said transparent material is a labile material of low viscosity at 180° C. or a liquid of low viscosity at 180° C.,   coating said solder preform with said predetermined thickness of said transparent material to produce a color coating by destructive interference, said transparent material having a melting point lower than the melting point of said solder preform.   
     
     
       21. The method of claim 20, wherein said transparent material comprises a poly-p-xylylene material. 
     
     
       22. The method of claim 21, wherein said poly-p-xylylene material has a melting point below 180° C. 
     
     
       23. The method of claim 22, wherein said poly-p-xylylene material is 4-ethyl-(2, 2) paracylophane. 
     
     
       24. The method of claim 21, wherein said predetermined thickness is in the range of about 50 nm to about 1000 nm. 
     
     
       25. The method of claim 24, wherein said predetermined thickness is in the range of about 60 nm to about 700 nm. 
     
     
       26. The method of claim 20, wherein said solder preform further includes a coating of flux on said preform, with said predetermined thickness of a transparent material effective to exhibit an interference color coated on said coating of flux. 
     
     
       27. The method of claim 26, wherein said transparent material comprises a poly-p-xylylene. 
     
     
       28. The method of claim 27, wherein said poly-p-xylylene is 4-ethyl-(2,2) paracylophane. 
     
     
       29. A method of distinguishing a first type of solder preform from a second type of solder preform, comprising the steps of: coating the first type of solder preform with a first predetermined thickness of a first transparent material effective to exhibit an interference color which produces a first color by destructive interference, said first transparent material having a melting point lower than the melting point of said first type of solder preform, wherein said first transparent material is a labile material of low viscosity at 180° C. or a liquid of low viscosity at 180° C.;   coating the second type of solder preform with a second predetermined thickness of a second transparent material effective to exhibit an interference color which produces a second color by destructive interference, said second transparent material having a melting point lower than the melting point of said second type of solder preform, wherein second said transparent material is a labile material of low viscosity at 180° C. or a liquid of low viscosity at 180° C.;   whereby said first and said second types of solder preforms are distinguishable by color.   
     
     
       30. The method of claim 29, wherein said transparent material comprises of poly-p-xylylene. 
     
     
       31. The method of claim 30, wherein said poly-p-xylylene has a melting point below 180° C. 
     
     
       32. The method of claim 31, wherein said poly-p-xylylene is 4-ethyl-(2,2) paracylophane. 
     
     
       33. The method of claim 31, wherein said poly-p-xylylene is 4-ethyl-(2,2) paracylophane. 
     
     
       34. The method of claim 30, wherein said predetermined thickness is in the range of about 50 nm to about 1000 nm. 
     
     
       35. The method of claim 34, wherein said predetermined thickness is in the range of about 60 nm to about 700 nm. 
     
     
       36. The method of claim 34, wherein said predetermined thickness is in the range of about 60 nm to about 700 nm. 
     
     
       37. The method of claim 30, wherein said poly-p-xylylene has a melting point below 180° C. 
     
     
       38. The method of claim 30, wherein said predetermined thickness is in the range of about 50 nm to about 1000 nm. 
     
     
       39. The method of claim 29, wherein said transparent material comprises of poly-p-xylylene. 
     
     
       40. A method of distinguishing a first type of solder preform from an uncoated solder preform, comprising the steps of: coating the first type of solder preform with a predetermined thickness of a transparent material effective to exhibit an interference color which produces a first color by destructive interference, said first color distinguishable from said uncoated solder preform having a second color, said transparent material having a melting point lower than the melting point of said first type of solder preform, wherein said transparent material is a labile material of low viscosity at 180° C. or a liquid of low viscosity at 180° C.